FIG. 1 illustrates a conventional axial water turbine 10 in an annular chamber 12 that may be embedded in the concrete of a water dam 13. An example of the axial turbine of the propeller blade type shown in FIG. 1, and particularly the mounting of the runner blades to the hub, is shown in U.S. Patent Application Publication 2009/0092496.
A water turbine is typically at an elevation well below the surface of the lake or other water source behind a dam. The annular chamber 12 defines a generally vertical axial water flow passage, as indicated by the arrow labeled H2O. Water flows through passages in the dam, through the annular chamber and over the runner blades. The blades and hub 16 rotate about a vertical axis. A shaft 24 extends between the hub and a generator. The force of the water rotates the runner blades, hub and shaft to drive the electric generator which produces electricity.
The hub 16 is tapered in the direction of water flow and may connect to a cone 18. The tapered surfaces of the hub and cone extend the hydraulic profile of the hub. Water flows smoothly over the hub and cone and down from the water turbine.
The more water flowing through the turbine, the greater the amount of power that can be delivered by the turbine to the generator. The amount of water flowing through the turbine is limited by the smallest cross-sectional area of the water passage in the turbine. This smallest cross-sectional area is typically the narrowest, e.g., the throat (T), portion of the annular water passage between the chamber 12 and hub 16. The throat is typically at the same elevation in the turbine as are the tips of the runner blades. The diameter of the throat (T) and the diameter (H) of the hub at the throat determine the smallest annular area of the water passage. The greater the ratio of the throat to the hub (T/H) the larger the water passage.
Runner blades are often made of stainless steel, which provides corrosion and cavitation resistance but is expensive. Where the hub is integrally cast with the runner blades, the hub is also formed of stainless steel. If the runner blades are not integrally formed with the hub, the hub may be formed of a less expensive material such as mild steel because the hub is less exposed to cavitation of the water than are the blades. Hubs formed of mild steel may be painted to provide corrosion resistance.
A common runner blade type is propeller-style blades bolted to a hub. This type of runner blade has a circular base 20 that bolts to a recessed circular mount 22 in the side of the hub, as is shown in FIG. 1. The bottom of the base has a mounting surface that is typically flat, circular and generally perpendicular to the axis of the runner blade.
FIG. 2 illustrates another conventional water turbine 26 having runner blades 28 fixed to a conical hub 30. Each runner blade 28 is integrally cast with a segment of the hub 30. The hub segments with the blades are arranged in an annular array and held together by shrink rings 32.
Water flows radially inward to the water turbine 26 and is discharged axially downward. The turbine 26 is a diagonal propeller blade turbine because the water moves diagonally across the runner blades. The chamber 34 extends cylindrically around the tips of runner blades. The conical hub 30 mounts to a head cover 36. The shaft 38 connects to the hub 30, extends through the head cover and connects to an electrical generator.
FIG. 3 illustrates another exemplary conventional water turbine 40. The propeller type runner blades 42 and hub 44 are formed as an integral metal casting. The water turbine 40 is an axial type turbine and is arranged in a chamber in a similar the chamber 12 for the water turbine 26 shown in FIG. 2.
The water turbines shown in FIGS. 1 to 3 illustrate conventional connections between the runner blades and hubs. As shown in these figures, runner blades may be integrally cast with the hub, welded to the hub, attached by shrink rings to the hub, or bolted to the hub. Integral casting, welding and in some applications of shrink rings are manufacturing and assembly steps applied at the manufacturing location for the water turbine. The water turbine is shipped with the blades attached to the hub to the dam where the turbine is to be installed. A difficulty arises when the water turbine is too large to be shipped from the manufacturing location to the water dam or other location at which the water turbine is to be installed.
An advantage offered by bolting the runner blades to the hub is that one or more of the blades may be attached to the hub at the water dam or other turbine installation location. Shipping the turbine hub by itself or with fewer than all of its runner blades reduces shipping costs and difficulties. Bolting the runner blades to the hub at the installation location saves transportation costs and reduces the difficulties associated with shipping a large water turbine. Similar to bolting the blades to the hub, the runner blades may be welded to the hub or secured with shrink rings at the installation location. Bolting, applying shrink rings and welding runner blades to hubs share the advantage of reducing the costs and difficulties of transporting water turbines.